1. Field of the Invention
The present invention concerns a new method combining evolution and rational design for the preparation of a micro-organism to produce 1,2-propanediol, the micro-organism thereby obtained and its use for the preparation of 1,2-propanediol.
2. Description of Related Art
1,2-propanediol or propylene glycol, a C3 dialcohol, is a widely-used chemical. It is a component of unsaturated polyester resins, liquid detergents, coolants, anti-freeze and de-icing fluids for aircraft. Propylene glycol has been increasingly used since 1993-1994 as a replacement for ethylene derivatives, which are recognised as being more toxic than propylene derivatives.
1,2-propanediol is currently produced by chemical means using a propylene oxide hydration process that consumes large amounts of water. Propylene oxide can be produced by either of two processes, one using epichlorhydrin, and the other hydroperoxide. Both routes use highly toxic substances. In addition, the hydroperoxide route generates by-products such as tent-butanol and 1-phenyl ethanol. For the production of propylene to be profitable, a use must be found for these by-products. The chemical route generally produces racemic 1,2-propanediol, whereas each of the two stereoisomers (R)1,2-propanediol and (S)1,2-propanediol are of interest for certain applications (e.g. chiral starting materials for specialty chemicals and pharmaceutical products).
The disadvantages of the chemical processes for the production of 1,2-propanediol make biological synthesis an attractive alternative. Two routes have been characterized for the natural production of 1,2-propanediol from sugars by microorganisms.
In the first route 6-deoxy sugars (e.g. L-rhamnose or L-fucose) are cleaved into dihydroxyacetone phosphate and (S)-lactaldehyde, which can be further reduced to (S)-1,2-propanediol (Badia et al, 1985). This route is functional in E. coli, but can not yield an economically feasible process due to the elevated cost of the deoxyhexoses.
The second route is the metabolism of common sugars (e.g. glucose or xylose) through the glycolysis pathway followed by the methylglyoxal pathway. Dihydroxyacetone phosphate is converted to methylglyoxal that can be reduced either to lactaldehyde or to acetol. These two compounds can then undergo a second reduction reaction yielding 1,2-propanediol. This route is used by natural producers of (R)-1,2-propanediol, such as Clostridium sphenoides and Thermoanaerobacter thermosaccharolyticum. 
Clostridium sphenoides has been used to produce 1,2-propanediol at a titer of 1.58 g/l under phosphate limited conditions (Tran Din and Gottschalk, 1985). Thermoanaerobacter thermosaccharolyticum has also been investigated for the production of 1,2-propanediol (Cameron and Cooney, 1986, Sanchez-Rivera et al, 1987). The best performances obtained were a titer of 9 g/l and a yield from glucose of 0.2 g/g. However, the improvement of the performances obtained with these organisms is likely to be limited due to the shortage of available genetic tools.
Cameron et al (1998) have investigated the use of E. coli as a platform for metabolic engineering for the conversion of sugars to 1,2-propanediol. Their theoretical analysis showed that the upper limit of the realistic product yield (considering mass balances and production of energy for growth) is significantly different depending on the culture conditions. Under anaerobic conditions, acetate will be produced as a by-product in order to recycle the reduced co-factors and the best yield shall be limited to 1 mole of 1,2-propanediol per mole of glucose (0.42 g/g). Under aerobic conditions, recycling of co-factors shall be ensured by the respiratory chain using oxygen as terminal electron acceptor and it could become possible to produce 1,2-propanediol without the production of by-products. Under these conditions, yield could reach at best 1.42 mol/mol (0.6 g/g). Considering the maximum titer of 1,2-propanediol, Cameron et al discussed its dependence on product and by-product toxicity. 1,2-propanediol is significantly less toxic than 1,3-propanediol and E. coli exhibits a residual growth rate of 0.5 h−1 with 100 g/l 1,2-propanediol. The inhibition of growth is more likely to be due to the by-product acetate that is known to be highly growth inhibiting. Development of an anaerobic process for the production of 1,2-propanediol with high titers and yields will have to address the acetate issue. Conversion of acetate into acetone, which is less inhibitory and easily removed in situ has been proposed (WO 2005/073364).
Several investigations for genetic modifications of E. coli in order to obtain a 1,2-propanediol producer using simple carbon sources have been done by the group of Cameron (Cameron et al, 1998, Altaras and Cameron, 1999, Altaras and Cameron, 2000) and the group of Bennett (Huang et al, 1999, Berrios-Rivera et al, 2003). These studies rely on the one hand on the expression of one or several enzymatic activities in the pathway from dihydroxyacetone phosphate to 1,2-propanediol and on the other hand on the removal of NADH and carbon consuming pathways in the host strain. The best results obtained by the group of Cameron are production of 1.4 g/l 1,2-propanediol in anaerobic flask culture with a yield of 0.2 g/g of glucose consumed. When extrapolated to an anaerobic fed-batch fermenter, the production was 4.5 g/l 1,2-propanediol with a yield of 0.19 g/g from glucose, far from the theoretical evaluation of Cameron et al. These performances have been obtained with the overexpression of the methylglyoxal synthase gene of E. coli (mgs), the glycerol dehydrogenase gene of E. coli (gldA) and the 1,2-propanediol oxidoreductase gene of E. coli (fucO) in a strain lacking the gene coding for lactate dehydrogenase (ldhA). Results obtained with the same approach but with lower titers and yields are also described in the patents U.S. Pat. No. 6,087,140, U.S. Pat. No. 6,303,352 and WO 98/37204.
The group of Bennett also used an E. coli host strain lacking ldhA for the overexpression of the mgs gene from Clostridium acetobutylicum and the gldA gene from E. coli. Flask cultures under anaerobic conditions gave a titer of 1.3 g/l and a yield of 0.12 g/g whereas microaerobic cultures gave a titer of 1.4 g/l with a yield of 0.13 g/g.
An alternative method to obtain a strain producing 1,2-propanediol is to direct the evolution of an “initial strain” towards a state where the “evolved strain” produces the desired compound with better characteristics. This method is based on the natural evolution of a microorganism which is first modified by attenuation of two genes, tpiA and one gene involved in the conversion of methylglyoxal into lactate. The purpose for attenuating the tpiA gene coding for triose phosphate isomerase is to separate the two metabolic branches starting at glyceraldehyde-3-phosphate (GA3P) and dihydroxyacetone phosphate (DHAP) that are normally interconverted by this enzyme. The pathway from DHPA to 1,2-propanediol will be the “reducing branch” consuming reduced co-factors (NADH), whereas the metabolism from GA3P to acetate will be the “oxidative branch” producing NADH and energy for the growth of the cell. Without a functional tpiA gene, the metabolism of the cell is “locked” and the growth of the strain, the production of 1,2-propanediol and the production of acetate are tightly coupled. Under selection pressure in an appropriate growth medium, this initial strain will evolve to a state where the production of 1,2-propanediol by said strain is improved. This procedure to obtain an “evolved strain” of micro-organism for the production of 1,2-propanediol is described in the patent application WO 2005/073364. This evolution process and the following step of fermentation are preferentially performed under anaerobic conditions. This technology is a clear improvement over the prior art. A 1,2-propanediol titer of 1.8 g/l was obtained, with a yield of 0.35 gram per gram of glucose consumed, close to the theoretical result of Cameron et al.
The object of the present invention is the improvement of an 1,2-propanediol producer strain by evolution and subsequent rational genetic engineering of the evolved strain. A special feature is the reconstruction of a functional tpiA gene in the evolved tpiA minus strain. These modifications lead to an improved production of 1,2-propanediol.